Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code

Thomas Gleixner
2020-03-01T18:12:50Z

Andy Lutomirski <luto@kernel.org> writes:
> On Sun, Mar 1, 2020 at 7:21 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>> Andy Lutomirski <luto@amacapital.net> writes:
>> >> On Mar 1, 2020, at 2:16 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> >> Ok, but for the time being anything before/after CONTEXT_KERNEL is unsafe
>> >> except trace_hardirq_off/on() as those trace functions do not allow to
>> >> attach anything AFAICT.
>> >
>> > Can you point to whatever makes those particular functions special? I
>> > failed to follow the macro maze.
>>
>> Those are not tracepoints and not going through the macro maze. See
>> kernel/trace/trace_preemptirq.c
>
> That has:
>
> void trace_hardirqs_on(void)
> {
> if (this_cpu_read(tracing_irq_cpu)) {
> if (!in_nmi())
> trace_irq_enable_rcuidle(CALLER_ADDR0, CALLER_ADDR1);
> tracer_hardirqs_on(CALLER_ADDR0, CALLER_ADDR1);
> this_cpu_write(tracing_irq_cpu, 0);
> }
>
> lockdep_hardirqs_on(CALLER_ADDR0);
> }
> EXPORT_SYMBOL(trace_hardirqs_on);
> NOKPROBE_SYMBOL(trace_hardirqs_on);
>
> But this calls trace_irq_enable_rcuidle(), and that's the part of the
> macro maze I got lost in. I found:
>
> #ifdef CONFIG_TRACE_IRQFLAGS
> DEFINE_EVENT(preemptirq_template, irq_disable,
> TP_PROTO(unsigned long ip, unsigned long parent_ip),
> TP_ARGS(ip, parent_ip));
>
> DEFINE_EVENT(preemptirq_template, irq_enable,
> TP_PROTO(unsigned long ip, unsigned long parent_ip),
> TP_ARGS(ip, parent_ip));
> #else
> #define trace_irq_enable(...)
> #define trace_irq_disable(...)
> #define trace_irq_enable_rcuidle(...)
> #define trace_irq_disable_rcuidle(...)
> #endif
>
> But the DEFINE_EVENT doesn't have the "_rcuidle" part. And that's
> where I got lost in the macro maze. I looked at the gcc asm output,
> and there is, indeed:

DEFINE_EVENT
DECLARE_TRACE
__DECLARE_TRACE
__DECLARE_TRACE_RCU
static inline void trace_##name##_rcuidle(proto)
__DO_TRACE
if (rcuidle)
....

> But I also don't see why this is any different from any other tracepoint.

Indeed. I took a wrong turn at some point in the macro jungle :)

So tracing itself is fine, but then if you have probes or bpf programs
attached to a tracepoint these use rcu_read_lock()/unlock() which is
obviosly wrong in rcuidle context.

Thanks,

tglx